This invention relates generally to the field of binocular vision testing and more specifically to a process for detecting disease by viewing rival contour elements.
During natural viewing, the two eyes send similar information to the brain concerning the images in the visual space. Images of different colors that occupy the same visual space can only be presented to the eyes by using artificial means such as a stereoscope, polarized images viewed through polarizing glasses, or complementary colored images viewed through lenses of complementary colors. When dissimilar but fusible images occupy the same visual space, rivalry ensues and the two dissimilar images compete for recognition. The two images may fuse to form a composite image or one rival image will be suppressed while the other image is seen. Images of different shapes will not fuse when their shapes differ too greatly.
Binocular rivalry has the potential usefulness of identifying conditions affecting the visual system, particularly if opposing rival endpoints are easily identifiable. Normally, nerve impulses traveling along the two optic nerves travel at the same velocity. When a disease affects neural transmission, the impulses from the defective eye travel at a slower velocity than the impulses from the healthy eye. When nerve impulses from one eye are conducted at a slower velocity than impulses from the opposite eye, a relative (one eye compared to the other eye) afferent (towards the brain or sensory) defect occurs. Due to slowing of the nerve conduction by the disease process, the nerve impulses on the side of the afferent defect will be delayed reaching the brain. The brain perceives the earlier arriving impulses (data of one rival component) from the healthy eye and suppresses the later arriving impulses (data of the opposing rival component) from the defective eye. A relative afferent defect can result from injury or disease of the eye, optic nerve, or brain. In amblyopia, the dysfunction responsible for the relative afferent defect is thought to be a dysfunction in processing of information by the brain.
Dimming the light to one of the two eyes can simulate a relative afferent defect. Similar to a disease process, dimming the light delays the nerve impulses traveling from that eye compared to the opposite brighter illuminated eye. Conversely, a simulated relative afferent defect can be measured by reducing the light to the brighter illuminated eye until the relative afferent defect is reversed. When disease affects one eye more than the other eye, the relative afferent defect can be counterbalanced and measured by dimming the light on the side of the healthy eye. This slows the nerve impulses originating from the healthy eye and allowing impulses from the defective (suppressed) eye to reach the brain first and be perceived.
Rivalry of black and white identically shaped images has been shown to produce a lustrous “gun metal” phenomenon by H. von Helmholtz (Handbuch der Physiologischen Optik, 1910). Greene (U.S. Pat. No. 4,863,258) demonstrated that by framing one image of an image pair of differing colors with a black border along two contiguous sides of the image or by circling the image with a black ring or border, the binocular perception would be a bright lustrous image. According to Greene, the perception of the dark border controls and determines the synthesis of information from the two eyes, and causes the stimulus material lying inside the boundary to dominate perception. Greene's invention is limited to the production of rival conditions by means of a dark border to produce the visual perception of luster. Hofeldt (U.S. Pat. No. 5,764,340) demonstrated that by using different colors as rival images without regard to borders, rivalry of colors could be used to measures visual function with the endpoint being the perception of one color over the complementary color without regard to the perception of luster.
Current techniques for distinguishing suppression from fusion of rival images require the subject to recognize the presence of luster (Greene) or identify different colors (Hofeldt). Both these endpoints are difficult concepts for children and some adults to comprehend. Luster is a subtle unnatural and dazzling phenomenon which requires education before it can be recognized. A rival test with easily identifiable images that distinguishes fusion from suppression would improve the usefulness of rivalry testing.
Current stereoscopes, for example the View-Master®, have a fixed interpupillary distance that fit either adults or children, but not both. These stereoscopes have one illuminated chamber and the illumination of the image pair is equal and uniform. There is no means for presenting scenes having areas differing in brightness. For example, in the modified View-Master® described by Hofeldt (U.S. Pat. No. 5,764,340), the reel contains neutral density filters and when engaged the entire scene dims which includes rival, binocularity, and stereoscopic stimuli. This global dimming on one side is an obvious clue to the subject of which eye is being suppressed.
Current stereoscopes are not used for viewing in both the upright and the inverted positions. Images for these stereoscopes are legible in only the upright position.